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On the more local scale, a deployment of seismometers around an individual glacier has provided insights on the seismic environment of a calving glacier, as well as the more immediate, short-term external drivers of calving events. We consider both local and global seismic data in order to further understanding of the dynamics of the calving process at Greenland outlet glaciers, and find that glacial earthquake production is indicative of a near-grounded terminus at the source glacier. We find that the locations derived from these events are accurate and are sensitive to changes in the calving-front position of the source glacier, and that the active-force azimuths are representative of the orientation of the glacier at the time of calving. We also find that these glaciers are the source of abundant small icequakes, which are strongly tied to the occurrence of major calving events. The small icequakes that occur at Helheim glacier are modulated by semi-diurnal variations in tide height, and potentially control the timing of major calving events by progressively damaging the glacier tongue.
I confirm the presence of a correlation between iceberg volume and glacial-earthquake size, which moves us closer to having the ability to use remotely recorded seismic signals to quantify mass loss at Greenland glaciers. This work presents testable hypotheses for future model development.
The occurrence of seismic events in glaciers has been an issue in the scientific literature since the early 1950s, following the report about icequakes in Baffin Island. Targeted seismological studies were undertaken by the Polish Expedition to Spitsbergen in 1962 and then continued at various glaciers in the Arctic, Antarctic and the Alps. The author of the book has been engaged in the project since 1970; he designed the layout of observations and instrumentation. The quakes he observed were categorized into two groups: typical seismic events called icequakes, and relatively long-period events named ice vibrations. In the case of icequakes, the space-time distributions and focal parameters were determined. In the case of ice vibrations, a spectral analysis was made. The present book is a synthesis of the results obtained. There are reports that the number of seismic events in glaciers has recently grown, which may be related to changing geometry of glaciers due to changing thermal conditions.
The mass balance of ice sheets is controlled by the dynamics of ice streams and glaciers which are in turn influenced by underlying geologic and geophysical processes. In this thesis, I use passive seismic measurements to improve constraints on glacial and lithospheric structures and to monitor temporal and spatial changes in glacier dynamic processes.Chapters 2 and 3 focus on imaging the crustal and upper mantle velocity structure of Greenland and investigating how subsurface properties may impact the long-term ice- sheet stability. To circumvent the limitations of a sparse network in a region with limited seismicity and improve upon the resolution of previous regional surface wave tomography models, I develop a new seismic processing approach that takes advantage of azimuthally well-distributed teleseismic events and available global dispersion models in order to expand regional data coverage and build path-specific dispersion curves. This global correction approach results in an unprecedentedly high-resolution group velocity model of Greenland (Chapter 2). Building upon this model, I combine ambient noise and earthquake data to derive a 3-D shear wave velocity model of Greenlands lithosphere from 10 to 200 km depth. Model uncertainties and robust model features are estimated by means of Bayesian inversion. The derived shear wave velocity model presents important structural details that correlate well with regional geologic features and shed light on the tectonic history of Greenland and its modern geologic environment (Chapter 3).Chapter 4 examines possible variations in Byrd glaciers dynamics via analysis of ice seismicity recorded during a summer field season. The results of this study indicate that, similar to other outlet glaciers, seismicity upstream of Byrd glacier is tidally- modulated and that seismicity detected in Byrds catchment is most likely associated with internal deformation of the firn layer. In chapter 4, I also explore the potential to use high frequency ambient seismicity to image ice structure and find that conventional seismic interferometry approach can be used even in regions with low ambient seismic noise levels which could have some positive implications for continuous seismic monitoring of subsurface structures at non-tidewater and dry polar glaciers.
The thickness of the Greenland Ice Cap was determined by seismic sounding along the trail from Camp Tuto to Camp Century in Greenland and on traverses northwest and southwest from Camp Century. The average velocity of vertically traveling seismic waves at each shot location was estimated using the first-arrival data from reflection records and the 10-m temperature at each location. The results of three long refraction profiles and measurements of temperature, density, and seismic velocities at the Camp Century drill hole were used to check velocity estimates. An empirical formula from Robin satisfactorily related seismic wave velocities to the temperature and density of the firn and ice. A two-layer glacier model having a homogeneous ice layer overlain by a firn layer in which the P-wave velocity increased linearly with depth was used. (Author).
"Jakobshavn Isbræ, a fast-flowing outlet glacier in West Greenland, began a rapid retreat in the late 1990's. The glacier has since retreated over 15 km, thinned by tens of meters, and doubled its discharge into the ocean. The glacier's retreat and associated dynamic adjustment are driven by poorly-understood processes occurring at the glacier-ocean interface. These processes were investigated by synthesizing a suite of field data collected in 2007-2008, including timelapse imagery, seismic and audio recordings, iceberg and glacier motion surveys, and ocean wave measurements, with simple theoretical considerations. Observations indicate that the glacier's mass loss from calving occurs primarily in summer and is dominated by the semi-weekly calving of full-glacier-thickness icebergs, which can only occur when the terminus is at or near flotation. The calving icebergs produce long-lasting and far-reaching ocean waves and seismic signals, including 'glacial earthquakes'. Due to changes in the glacier stress field associated with calving, the lower glacier instantaneously accelerates by ~3% but does not episodically slip, thus contradicting the originally proposed glacial earthquake mechanism. We furthermore showed that the pre-dominance of calving during summer can be attributed to variations in the strength of the proglacial ice mélange (dense pack of sea ice and icebergs). Sea ice growth in winter stiffens the mélange and prevents calving; each summer the mélange weakens and calving resumes. Previously proposed calving models are unable to explain the terminus dynamics of Jakobshavn Isbræ (and many other calving glaciers). Using our field observations as a basis, we developed a general framework for iceberg calving models that can be applied to any calving margin. The framework is based on mass continuity, the assumption that calving rate and terminus velocity are not independent, and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset. Although the calving framework does not constitute a complete calving model, it provides a guide for future attempts to define a universal calving law"--Leaf iii.
Glacier ice responds to environmental forcing through changes in its sliding speed and mass balance. While these changes often occur on daily time scales or longer, they are initiated by brittle deformation events that establish hydrological pathways in hours or seconds and allow meltwater access to englacial or subglacial depths to facilitate ice motion. In this thesis, we (various contributing authors including myself) use seismic monitoring to detect and locate the creation and growth of some of these hydraulic pathways by monitoring their seismic emissions, or icequakes. More specifically, we address (1) what seismic observables, unavailable from other sensing methods, indicate an initial glaciogenic response to melt- water input and (2) if these comprise evidence of feedbacks that may destabilize polar ice under a warming climate. Supplemental to our scientific contributions, we advance statistical processing methods that demonstrably improve the capability of digital detectors at discriminating icequakes from astationary noise. We begin by interpreting geophysical observations collected from a dry-based, sub-freezing (-17° C), polar glacier environment (Taylor Glacier, ANT). By implementing a calibrated surface energy balance model, we estimate the timing and volume of surface meltwater generated during the collection of seismic data from a six-receiver geophone network. This comparison illustrated that any effectively nonzero meltwater triggered large, repeating icequakes localized near a deep, supraglacial-to-subglacial crack within a melt-water catchment region. The focal mechanisms of these icequakes are consistent with an expansive growth within the crack. Their occurrence at night suggests that this expansion was accommodated by volumetric straining of confined, re-freezing meltwater. These cracks likely sustained their surface-to-bed hydrological connection, in the absence of melt-assisted basal sliding. Further, this appears to be the first report attributing seismogensis in glacial ice to fracturing induced by phase change. We proceed by contrasting these response characteristics with geophysical observations following an early (spring) supraglacial lake drainage within the lake-forming ablation zone of the Western Greenland Ice Sheet. The subglacial drainage system present during this time was considered incapable of efficiently draining large fluxes of meltwater input and therefore likely to undergo transient motion. Using measurements from a ~5km-aperture geophone network, we find that the anticipated post-drainage icequakes are diurnally responsive, largely surficial in origin, and indicative of tensile fracturing from shallow cracks in the ice. The creation of the lake-drainage moulin appears to coincide with a shift in mean icequake source locations, and an increase in icequake occurrence at night relative to that in the day. Contrary to our expectations, we find that the timing of GPS-derived surface speeds do not clearly indicate this seismic activity on any given day. Rather, these icequakes are best explained by peaks in localized strain gradients that develop at night when decreased subglacial water flux likely increases variability in basal traction. Additionally, our results appear comprise the first detailed seismic observations targeted at an actively draining lake. Our last study addresses the apparent deficiency in observed basal icequakes detected from Greenland lake site. To explain the lack of deep icequakes, we compute thresholds on the magnitude of detectable basal events within the network and thereby illustrate that surficial icequakes with similar magnitudes and spectral content are more likely to be observed. By restricting our attention to seismic events that produce lower frequency waveforms, we find a population of nearly monochromatic, sub-1Hz, large magnitude ( M[subscript w] [less than or equal to] 3) seismic events borne from remote glaciogenic sources. In contrast to surficial icequakes, these events occur without significant bias between day and/or night periods and are best explained as glacial earthquakes generated by sliding episodes or iceberg calving events in the vicinity of Jakobshavn Glacier. These events occur daily and not correlate with the presence of local, surficial seismicity. We conclude with three general assertions regarding melt-triggered response characteristics of polar ice. First, hydraulic connections established by fracture events do not necessarily result in seismogenic basal stick slip, and therefore cannot necessarily be observed with conventional GPS monitoring. This was demonstrated at Taylor Glacier. Here, meltwater input to a hydraulic pathway led to fracture growth deep within a cold glacier without any change in surface speed. Second, the presence of melt-triggered basal sliding does not necessarily induce a clear seismogenic basal response in the lakes regions. This was demonstrated on the Greenland Ice Sheet. Seismogenesis may instead be more clearly reflected by surficial strain gradients established by variability in basal traction, suggesting these feedbacks are secondary rather than primary. The response is therefore not clearly indicated from day-to-day timing of GPS-observations. Third, the absence of an observed local response does not necessarily indicate the absence of a local physical response. This was also illustrated in Greenland. Here, deep local icequakes are likely muted by noise, waveform-attenuating ice, and viscous basal rheology. Magnitude thresholds suggest that M[subscript w][less than or equal] 2 for consistent recording of local, basal sources. In contrast, remote, low frequency seismic events were clearly observed, and attributed to activity within ice catchments along the western edge of the ice sheet or Jacobshavn glacier. Finally, we assert that early-indicators of melt-triggered glacial response include components of spatially localized, brittle deformation that is most suitable to seismic observation. Critically-stable regions along mass-balance equilibrium lines constitute potential sites for newly forming surface-to-bed hydraulic connections in a warming climate, and likewise, a potential target for future seismic experiments.
An Introduction to Seismology, Earthquakes and Earth Structures is an introduction to seismology and its role in the earth sciences, and is written for advanced undergraduate and beginning graduate students. The fundamentals of seismic wave propagation are developed using a physical approach and then applied to show how refraction, reflection, and teleseismic techniques are used to study the structure and thus the composition and evolution of the earth. The book shows how seismic waves are used to study earthquakes and are integrated with other data to investigate the plate tectonic processes that cause earthquakes. Figures, examples, problems, and computer exercises teach students about seismology in a creative and intuitive manner. Necessary mathematical tools including vector and tensor analysis, matrix algebra, Fourier analysis, statistics of errors, signal processing, and data inversion are introduced with many relevant examples. The text also addresses the fundamentals of seismometry and applications of seismology to societal issues. Special attention is paid to help students visualize connections between different topics and view seismology as an integrated science. An Introduction to Seismology, Earthquakes, and Earth Structure gives an excellent overview for students of geophysics and tectonics, and provides a strong foundation for further studies in seismology. Multidisciplinary examples throughout the text - catering to students in varied disciplines (geology, mineralogy, petrology, physics, etc.). Most up to date book on the market - includes recent seismic events such as the 1999 Earthquakes in Turkey, Greece, and Taiwan). Chapter outlines - each chapter begins with an outline and a list of learning objectives to help students focus and study. Essential math review - an entire section reviews the essential math needed to understand seismology. This can be covered in class or left to students to review as needed. End of chapter problem sets - homework problems that cover the material presented in the chapter. Solutions to all odd numbered problem sets are listed in the back so that students can track their progress. Extensive References - classic references and more current references are listed at the end of each chapter. A set of instructor's resources containing downloadable versions of all the figures in the book, errata and answers to homework problems is available at: http://levee.wustl.edu/seismology/book/. Also available on this website are PowerPoint lecture slides corresponding to the first 5 chapters of the book.